The invention relates to a liquid crystal display device comprising a liquid crystalline medium between a supporting plate, and a cover plate at least one of which plates is transparent and each of which is at least provided with a layer comprising a conducting material and, if necessary, a layer of protective material on the side of the liquid crystalline medium.
The invention also relates to a method of manufacturing such a display device.
Such a display device is used, for example, in colour television or in (colour) monitors for data display and in automotive dashboards, etc., but they may also be used as light switches in optical equipment or other optical applications. In addition, such devices are increasingly being used in projection television.
The supporting plate and the cover plate are generally in the form of two glass substrates on which electrodes (metal patterns) are provided. These substrates have a given thickness to render them mechanically robust, which is necessary because during assembly spacers are provided between the plates and the plates are pressed against the spacers by applying sub-atmospheric pressure.
Usually fibres or spheres of the desired dimensions, which are deposited in advance on one of the plates, are chosen for the spacers. Since a certain spread occurs in the diameter of these fibres or spheres, there will be a local spread in the thickness of the liquid crystal layer. This spread is increased by unevennesses in the supporting plate and/or the cover plate owing to the presence of metal tracks and electronic switching elements such as diodes and transistors. A surface unevenness of the order of 0.5 .mu.m is no exception. Especially at smaller cell thicknesses (2 to 3 .mu.m) this variation will play a considerable role.
A partial solution to this problem is proposed in German Offenlegungsschrift 35,29,581. As described therein, after completion of the supporting plate, the spacers are defined by coating the plate with a uniform layer of, for example, a resin or photoresist and an insulating layer, whereafter the spacers are formed photolithographically from this double layer. A second supporting plate or cover plate is subsequently laid on the spacers while simultaneously maintaining a sub-atmospheric pressure in the space corresponding to the cell to be formed.
Various problems occur in such a manufacturing method. In the first place, the two supporting plates must be correctly positioned with respect to each other. Moreover, particularly for small cell thicknesses, it is difficult to fill the devices with liquid crystal material.
In the above method, it also appears that there is still a variation in the thickness of the liquid crystal layer which may be up to 10% or more of the total layer thickness. For a large part, these differences in thickness are caused by the fact that unevennesses in the one supporting plate are not compensated by off-setting variations in the other supporting plate.